21 Second Backup by Charles Leborgne was quite an amazing hardware / software combo back in the day.  I always wanted to have this when I was a kid, but somehow ended up with the Datel Burst Nibbler combo instead.  I recently picked up a reproduction of this hardware from COREi64.  For a long time, the software side of this combo was uncopiable, but recently that has changed.  Here is a YouTube video from 8-Bit Resurgence demonstrating that it was indeed copied.

Here is a LINK to an interview with the author of 21 Second Backup.

Here is a LINK to the C64 Copy Protection website that has more details about the program and even a diagram of how to build the custom hardware cable yourself.